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The majority of stars reside in multiple systems, especially binaries. Theformation and early evolution of binaries is a longstanding problem in starformation that is not fully understood. In particular, how the magnetic fieldobserved in star-forming cores shapes the binary characteristics remainsrelatively unexplored. We demonstrate numerically, using the ENZO-MHD code,that a magnetic field of the observed strength can drastically change two ofthe basic quantities of a binary system: the orbital separation and mass ratioof the two components. Our calculations focus on the protostellar massaccretion phase, after a pair of stellar 'seeds' have already formed. We findthat, in dense cores magnetized to a realistic level, the angular momentum ofthe gas accreted by the protobinary is greatly reduced by magnetic braking.Accretion of strongly braked material shrinks the protobinary separation by alarge factor compared to the non-magnetic case. The magnetic braking alsochanges the evolution of the mass ratio of unequal-mass protobinaries byproducing gas of low specific angular momentum that accretes preferentiallyonto the primary rather than the secondary. This is in contrast with thepreferential mass accretion onto the secondary previously found forprotobinaries accreting from an unmagnetized envelope, which tends to drive themass ratio towards unity. In addition, the magnetic field greatly modifies themorphology and dynamics of the protobinary accretion flow. It suppresses thecircumstellar and circumbinary disks that feed the protobinary in thenon-magnetic case; the binary is fed instead by a fast collapsing pseudodiskwhose rotation is strongly braked. The magnetic braking-driven inward migrationof binaries from their birth locations may be constrained by high-resolutionobservations of the orbital distribution of deeply embedded protobinaries,especially with ALMA.

Orbital and Mass Ratio Evolution of Protobinaries Driven by Magnetic Braking